The production of foams from propylene polymers is hampered by the low melt viscosity and melt elasticity of these polymers. While attempting to use standard propylene resins for making foams this usually results in a lack of stability of the foam bubbles or foam cell walls and consequently collapsed or poor quality foams.
DeNicola et al. disclose in U.S. Pat. No. 5,605,936 that propylene polymers of increased melt strength can be obtained by irradiation of the polymers. However, when such polymers are blended with elastomers to obtain better impact strength, the melt strength suffers. Irradiation of heterophasic (or impact) resins by conventional radiation methods yielded improved melt strength resins but resulted in severe gelation and high degree of crosslinking occurs which are undesirable in foaming applications (column 1, lines 42-58 and Control Examples 6-8). DeNicola et al. suggest to provide foams from a composition of three different components, including high levels (30-80 weight percent) of ethylene-propylene copolymers elastomer which blend is typically subjected to irradiation.
In U.S. Pat. No. 5,348,795 Park discloses open-cell foams of low density which are dimensionably stable and typically prepared from high melt strength propylene homopolymers or high melt strength propylene random copolymers.
In co-pending U.S. patent application Ser. No. 09/133,576 filed Aug. 13, 1998 and WO-99/10424 in-situ rheology modification of polyolefins is disclosed as applied to, among others, polypropylenes and blends of polypropylenes with elastomers resulting in polypropylenes having advantageous melt processing properties. Foams are mentioned among the possible end uses or applications of such rheology-modified propylene polymers. However, there is no further specific teaching on materials suitable for foams or foam properties.
There is still a demand for foams which are non-brittle and have good low temperature cushioning properties. It is also desirable to produce such foams using uncomplicated foaming processes and equipment.
The invention in one aspect provides a foam comprising a propylene copolymer material comprising at least 50 weight percent of units derived from propylene, based on the total propylene copolymer material, and units derived from ethylenically unsaturated comonomers and having a melt flow rate in the range of from 0.5 to 8 g/10 min, a melt strength of at least 5 cN, and a melt drawability of at least 20 mm/s.
In addition there is provided a process for making a foam, which process comprises converting a propylene copolymer material comprising at least 50 weight percent of units derived from propylene, based on the total propylene copolymer material, and units derived from ethylenically unsaturated comonomers and having a melt flow rate in the range of from 0.5 to 8 g/10 min, a melt strength of at least 5 cN, and a melt drawability of at least 20 mm/s, optionally mixed with a nucleating agent, into a polymer melt, introducing, at an elevated pressure, at least one blowing agent into the polymer melt to form a foamable gel, cooling the foamable gel, and extruding the foamable gel into a region of lower pressure to form a foam.
According to a further aspect the invention provides a process for making a foam in the form of thermoplastic foam beads, which process comprises converting a propylene copolymer material comprising at least 50 weight percent of units derived from propylene, based on the total propylene copolymer material, and units derived from ethylenically unsaturated comonomers and having a melt flow rate in the range of from 0.5 to 8 g/10 min, a melt strength of at least 5 cN, and a melt drawability of at least 20 mm/s, optionally mixed with a nucleating agent, into a polymer melt, cooling and granulating the polymer melt to form discrete resin particles, creating a suspension by dispersing the resin particles in a liquid medium in which they are substantially insoluble, introducing, at an elevated temperature and pressure, at least one blowing agent into the suspension to form resin particles having a blowing agent incorporated therein, and rapidly discharging the product into an atmosphere that promotes converting the product into foam beads.
Further the invention provides an expandable composition comprising a propylene copolymer material comprising at least 50 weight percent of units derived from propylene, based on the total propylene copolymer material, and units derived from ethylenically unsaturated comonomers and having a melt flow rate in the range of from 0.5 to 8 g/10 min, a melt strength of at least 5 cN, and a melt drawability of at least 20 mm/s, optionally mixed with a nucleating agent, and a blowing agent.